Determining the sites of synthesis and the conditions affecting the secretion of growth factors, e.g., of mouse epidermal growth factor (mEGF), requires highly sensitive assays. We have, therefore, established conditions for the use of the ultrasensitive enzymatic radioimmunoassay (USERIA) to detect and quantitate such factors. Highly purified mEGF and anti-mEGE-IgG were reacted in polyvinyl microtiter plates, and the sensitivity of USERIA and the enzyme-linked immunoabsorbent assay (ELISA) were compared. The USERIA, a modification of the ELISA in which tritiated adenosine-5'-monophosphate (3H-AMP) is substituted for p-nitrophenyl phosphate (pNPP), a colorimetric substrate, was shown to be superior to the ELISA in both noncompetitive and competitive assays. As little as 8 fmole mEGF has been successfully used as the solid-phase competitor for the USERIA. Experiments are now underway to determine the applicability of the competitive USERIA for measurement of growth factors in human cells.